This invention relates generally to imaging, and more particularly to determining the mass and the volume of an object or objects in reconstructed images. Configurations of the present invention are particularly useful in medical and diagnostic computed tomographic (CT) applications for quantification of calcification and/or lesions, but the present invention is not limited to medical applications or to CT.
In some known CT imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units” (HU), which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two-dimensional slice taken through the object.
To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical, weighting scheme, a method can be derived to multiply the weight with the projection data prior to the filtered backprojection algorithm.
Coronary artery disease (CAD) is a leading cause of death in the developed world. One known diagnostic imaging exam for the diagnosis of CAD is coronary angiography, which can be used to detect blockages or obstructions in coronary arteries resulting from buildup of plaque. Coronary angiography is an invasive exam, and its application to a large asymptomatic population for the purpose of earlier detection of the disease is impractical. However, coronary artery calcification (CAC) is a good indicator of the presence of plaque. CAC can be imaged using non-invasive methods like computed tomography (CT) imaging.
Imaging the heart poses special difficulties due to its constant motion. An imaging modality used to isolate the motion of a heart requires an acquisition speed of less than 50 ms per slice. Angiography is ideally suited for imaging blockages in the coronary arteries because it provides an acquisition speed of less than 10 ms. Known CT image acquisition systems that provide prospective and retrospective cardiac gating cardiac gating using electrocardiogram (ECG) signals have acquisition speeds that approach about 100 ms. This speed is sufficient to freeze the left portion of a heart at end diastole for imaging the left coronary arteries, which are the arteries of most concern in CAD detection. Known CT imaging systems using cardiac gating make it possible to image a heart non-invasively to determine calcification content of coronary arteries.
A known clinical cause for coronary calcification is a “healing” process of a vessel after its weakening due to a buildup of plaque. There are three stages of calcification of a vessel, namely, a) a completely healed vessel, b) a partially healed vessel, or c) a vessel that has just started the healing process. A vessel may also be between two of these stages. At least some known algorithms for calcification determination utilize AJ, mass score and/or volume score to determine calcification content of arteries. However, variability in reproducibility of the calcification mass and volume results of the known algorithms has been cited as a reason for using CAC only as a very specific negative test.